Modulating Stepwise Photochromism in Platinum(II) Complexes with

Oct 14, 2013 - Apart from the character of the linker that spaces the photochromic units, it is ..... At the PSS, only one P signal at 11.98 ppm was o...
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Modulating Stepwise Photochromism in Platinum(II) Complexes with Dual Dithienylethene−Acetylides by a Progressive Red Shift of RingClosure Absorption Bin Li,† Hui-Min Wen,† Jin-Yun Wang,† Lin-Xi Shi,† and Zhong-Ning Chen*,†,‡ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China S Supporting Information *

ABSTRACT: To modulate stepwise photochromism by shifting ring-closure absorption of the dithienylethene (DTE) moiety, trans-Pt(PEt3)2(CC-DTE)2 [CC-DTE = L1o (1oo), L2o (2oo), L3o (3oo), and L4o (4oo)] and cisPt(PEt3)2(L4o)2 (5oo) with two identical DTE−acetylides were elaborately designed. With the gradual red shift of ringclosure absorption for L1c (441 nm) → L2c (510 nm) → L3c (556 nm) → L4c (602 nm), stepwise photochromism is increasingly facilitated in trans-Pt(PEt3)2(CC-DTE)2 following 1oo → 2oo → 3oo → 4oo. The conversion percentage of singly ring-closed 2co−4co to dually ring-closed 2cc−4cc at the photostationary state is progressively increased in the order 1cc (0%) → 2cc (18%) → 3cc (67%) → 4cc (100%). Compared with trans-arranged 4oo, stepwise photochromism in the corresponding cis-counterpart 5oo is less pronounced, ascribed to either direct conversion of 5oo to 5cc or rapid conversion of 5co to 5cc. The progressively facile stepwise photocyclization following 2oo → 3oo → 4oo is reasonably interpreted by gradually enhanced transition character involving LUMO+1, which is the only unoccupied frontier orbital responsible for further photocyclization of singly ring-closed 2co−4co.

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INTRODUCTION Photochromic systems with the dithienylethene (DTE) moiety have been widely applied in photonics, electronics, and material and biological fields.1−3 Thermal stability, fatigue resistance, and convenient functionalization make them excellent components for memories and switches at the molecular level. On the one hand, the incorporation of DTE to a suitable metal ion through coordination bonds is a feasible approach for improving the photochromic performance as well as modulating intramolecular electronic and magnetic interactions.4−22 On the other hand, because the integrated entities with two or more DTE units are useful in achieving multiswitchable, multicolor, and multistate systems, multiphotochromism is highly promising for multifrequency optical memories to store more information in a single molecule as well as build complex devices.23 Nevertheless, achieving stepwise or selective photochromism in a multi-DTE system to access all of the possible ringopened/closed isomers is quite challenging because ring closure in one DTE moiety must be well communicated with other ring-opened photochromes, while still retaining the photochemical reactivity of each DTE unit. On the one hand, ring closure at one DTE moiety usually impedes further photo© XXXX American Chemical Society

reactivity of other DTE units because of rapid intercomponent energy transfer from the ring-opened moiety to the ring-closed one so that only partially ring-closed forms could be obtained, whereas a fully ring-closed isomer is normally inaccessible.24−32 On the other hand, for electronically isolated systems with complete independence for each DTE unit, irradiation with UV light usually induces simultaneous photocyclization for each DTE moiety so that mixed ring-closed/opened forms could not be accessed.33−40 In this case, simultaneous ring closure results simply in cumulative absorption spectra of multi-DTE units without emergent features or spectral shifts. To achieve stepwise photochromism in a molecule combined with multi-DTE moieties, it is necessary to facilitate electronic interaction but minimize energy transfer from ring-opened DTE moieties to ring-closed ones by the judicious selection of a suitable connector to space DTE units while retaining the electronic communication between them. 1,4-Phenylene,41 dimethylsilyl,42 or 1,3,5-trivinylphenylene43 seems to be a suitable spacer for stepwise photochromism when multi-DTE units are proximately connected. Complexation of multi-DTE Received: June 22, 2013

A

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Figure 1. (a) Photochromic reactions and color changes of L1o−L4o. (b) UV−vis absorption spectra of L1c−L4c in CH2Cl2.

absorption usually shows a distinct blue shift relative to the DTEs having 3-thienyl.47 L1o with two 2-thienyl moieties exhibits ring-closure absorption centered at 441 nm. By comparison, ring-closure absorption of L2o having one 2thienyl and one 3-thienyl shows a distinct red shift to 510 nm. Ring-closure absorption of L3o with two 3-thienyl moieties is further red-shifted to 556 nm. Compared with that of L3o, ringclosure absorption of L4o is distinctly red-shifted to 602 nm upon introduction of 2-pyridyl. As depicted in Figure 1, ringclosure absorption maxima occur at 441, 510, 556, and 602 nm upon irradiation of L1o−L4o at 365 nm to the photostationary state (PSS), with the colorless solution turning yellow, red, purple, and blue, respectively. It is worth mentioning that the counterpart of L1o with 5-(trimethylsilyl)ethynyl instead of 4(trimethylsilyl)ethynyl shows inactive photoreactivity. Photochemical quantum yields and conversion percentages of L1o− L4o at the PSS are summarized in Table 1.

moieties with a metal ion is an alternative approach for stepwise photochromism,44−46 although it is undetectable in most cases.30−32,39 When two DTE units are bound to an AuI,44 PtII,45 or RuII/III46 through bis(σ-acetylide) bonds, distinct stepwise photochromism has been indeed observed, in which ring closure at one DTE unit does not significantly reduce the photochemical reactivity at the other. It is worth mentioning that when two DTE units are bound to a Ru(dppe)2 [dppe = 1,2-bis(diphenylphosphino)ethane] moiety through the formation of two Ru−acetylide σ bonds, distinct stepwise photochromism can be achieved by different stimuli such as light irradiation or electrochemical oxidation/reduction.38,46 With a theoretical computational approach, it has been proposed by Jacquemin and co-workers that only the unoccupied frontier orbitals that contribute to UV absorption in the mixed ring-opened and -closed isomers are responsible for further photocyclization toward the formation of a fully ring-closed form.23 Apart from the character of the linker that spaces the photochromic units, it is likely that DTE itself plays a crucial role in achieving stepwise photochromism. With this in mind, we are devoted to modulating ring-closure absorption of the DTE moiety by modifying the π-conjugated system. When cyclopentene is integrated with thienyl in different positions and 2-pyridyl is introduced, DTE−ethynyl ligands L1o−L4o (Figure 1) are elaborately designed, which show a progressive red shift of ring-closure absorption in the order L1c (441 nm) → L2c (510 nm) → L3c (556 nm) → L4c (602 nm). Remarkably, with the progressive red shift of ring-closure absorption in DTE−ethynyl ligands, stepwise photochromism is increasingly pronounced in the corresponding trans-Pt(PEt3)2(CC-DTE)2, following 1oo → 2oo → 3oo → 4oo (100%) (Scheme S1 in the Supporting Information, SI). Furthermore, trans-platinum(II) complex 4oo is more favorable for stepwise photochromism than the cis-oriented counterpart 5oo.

Table 1. Photochemical Quantum Yields and Conversion Percentages at the PSS for L1o−L4oa Φo→cc L1o L2o L3o L4o

0.62 0.38 0.22 0.40

(→L1c) (→L2c) (→L3c) (→L4c)

Φc→od

conversion at the PSS (%)b

0.048 (→L1o) 0.86 (→L2o) 0.082 (→L3o) 0.027 (→L4o)

58 48 45 >95

Data obtained with an uncertainty of ±10%. b Conversion percentages measured by NMR spectroscopy. cData obtained by irradiation at 365 nm. dData obtained by visible-light irradiation at >460 nm. a

trans-Pt(PEt3)2(CC-DTE)2 complexes 1−4 were prepared by the reaction of trans-Pt(PEt3)2Cl2 with 2 equiv of L1o−L4o through CuI-catalyzed Pt−acetylide bonding formation. Similarly, the cis-Pt(PEt3)2(L4o)2 complex 5 was accessible by the same synthetic procedure using cis-Pt(PEt3)2Cl2 and L4o. It is noteworthy that cis-oriented complex 5 is sufficiently stable under UV−vis irradiation without conversion to the trans isomer or degradation. UV−Vis Absorption Spectral Studies. Complexes 1oo− 5oo exhibit intense absorption bands at ca. 260−360 nm because of intraligand (IL) transitions within two DTE− acetylides together with absorption shoulder bands tailing to

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RESULTS AND DISCUSSION Considering that ring-closure absorption of DTE is quite sensitive to the combining position of the thienyl moiety to the cyclopentene moiety, the DTE−ethynyl ligands L1−L4 with various π-conjugated systems are elaborately designed. When 2thienyl is attached to the cyclopentene moiety, ring-closure B

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450 nm, arising mainly from 5d(Pt) → π*(CC-DTE/PEt3) metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) states from one ring-opened CCDTE to another, as revealed by time-dependent density functional theory (TD-DFT) studies (vide infra). When 1oo in CH2Cl2 was irradiated under UV light at 365 nm, the intense absorptions at 288 and 332 nm decreased gradually, whereas a new shoulder band at ca. 450 nm occurred because of photocyclization of L1o to L1c (Figure 2a).

2b) occurred and progressively enhanced in the intensity. The reaction reached the PSS in 80 s, with the colorless solution turning red. Because of the low conversion percentage to 2cc (18%) at the PSS as demonstrated by NMR studies (vide infra), a distinct shift of the ring-closure band was unobserved with the ongoing stepwise photocyclization 2oo → 2co → 2cc. When a CH2Cl2 solution of 3oo was irradiated at 365 nm, the intense absorption bands at 252, 296, and 355 nm reduced gradually. Meanwhile, the ring-closure absorption maximum occurred at 567 nm (Figure 2c), which showed a gradual red shift to 585 nm at the PSS, with the colorless solution becoming purple. Such a distinct red shift of ring-closure absorption demonstrated unambiguously that stepwise photocyclization occurred indeed through 3oo → 3co → 3cc. A 543 cm−1 (18 nm) red shift of the ring-closure absorption upon first the formation of 3co (567 nm) and then 3cc (585 nm) is ascribed to the increased π system in the latter. Conversely, when the solution at the PSS was irradiated with light at >460 nm (Figure S1 in the SI), the reversed UV−vis absorption spectral changes were observed with stepwise cycloreversion reactions 3cc → 3co → 3oo. Moreover, the reversibility of the photochromic behavior on complex 3oo has been studied. As shown in Figure 3, 3oo shows good reversibility in its

Figure 2. UV−vis absorption spectral changes of 1oo (a), 2oo (b), 3oo (c), 4oo (d), and 5oo (e) in a CH2Cl2 solution at ambient temperature upon irradiation at 365 nm.

Figure 3. UV−vis absorbance changes of complex 3 at 585 nm upon alternate excitation at 365 and >460 nm over six cycles at room temperature.

Nevertheless, the absorbance at ca. 450 nm is very low even if the solution was irradiated at 365 nm for quite a long time. The quite low ring-closure conversion percentage (0.60). In contrast, ring-closure absorption of 1co due to HOMO → LUMO+1 displays a very weak oscillator strength (0.07), coinciding with the quite low photocyclization efficiency of 1oo with less than 5% conversion to 1co. It has been proposed that only the unoccupied frontier orbitals contributing to the UV absorption bands participate in photocyclization.23,45b For 2oo−5oo species, both LUMO and LUMO+1 are responsible for ring-closure reaction. For singly ring-closed 2co−5co species, the LUMO is generally localized on the ring-closed DTE unit, which is irrelevant with further photocyclization at the other ring-opened DTE moiety. Instead, LUMO+n that exhibit good bonding character and significant density at the reactive C atoms for the to-be-formed C−C bond in the ring-opened DTE moiety23 are mostly responsible for further ring closure of 2co−5co to produce 2cc−5cc. In fact, LUMO+1 represents the only photochromism-favorable orbital that is mainly resident on the ring-opened DTE moiety in 2co−5co. As depicted in Figure 7, although LUMO+1, LUMO+2, and LUMO+3 of 3co are resident on the ring-opened DTE unit to F

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opened DTE. In contrast to distinct stepwise photochromism for 4oo → 4co → 4cc, a much lower conversion maximum of cis-arranged 5co (19%) was detected because of either rapid conversion of 5co to 5cc or direct conversion of 5oo to 5cc. As a result, trans-Pt(PEt3)2 as a spacer is more favorable than cisPt(PEt3)2 for stepwise photochromism because it transmits more pronounced electronic interaction. The results offer a new approach to modulating stepwise photochromism through modifying ring-closure absorption of DTE moieties apart from altering the linker.

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Figure 7. Plots of unoccupied orbitals involved in the absorption transitions for singly ring-closed 3co. The red and green regions donate different phases. The reactive C atoms in LUMO+1 responsible for further ring closure are indicated by arrows.

EXPERIMENTAL SECTION

General Procedures and Materials. All of the synthetic procedures were carried out using Schlenk techniques and vacuumline systems under a dry argon atmosphere. Solvents were distilled under an argon atmosphere in the presence of sodium benzophenone or calcium hydride. cis/trans-Pt(PEt3)2Cl2, 1-(3,5-dimethyl-2-thienyl)2-(4-bromo-3,5-dimethyl-2-thienyl)perfluorocyclopentene, 3-bromo-2methyl-5-(trimethylsilyl)ethynylthiophene, (3,5-dimethyl-2-thienyl)perfluorocyclopentene, (2,5-dimethyl-3-thienyl)perfluorocyclopentene, and L4o were prepared by literature procedures.46,47,51,52 Synthesis of 1-(3,5-Dimethyl-2-thienyl)-2-(4-iodo-3,5-dimethyl-2-thienyl)perfluorocyclopentene. A solution of 1-(3,5d imethy l-2-th ieny l)-2- (4-bromo- 3,5-dimethyl- 2-thienyl) perfluorocyclopentene (948 mg, 2 mmol) in anhydrous ether (40 mL) was cooled to −78 °C. To the solution was slowly added nbutyllithium (1.6 M in hexane, 1.3 mL, 2.1 mmol). After stirring for 1 h at −78 °C, a solution of iodine (1.03 g, 4 mmol) in anhydrous ether (10 mL) was added to the reaction solution. The mixture is then stirred at −78 °C for 2 h and then slowly warmed to room temperature with stirring for 16 h before the addition of a solution of aqueous sodium sulfite (30 mL). The mixture was extracted with petroleum ether three times. The combined organic layer was dried with MgSO4, then filtered, and evaporated in vacuo. The product was purified by column chromatography on silica gel using petroleum ether. Yield: 70% (730 mg). 1H NMR (CDCl3, ppm): δ 6.52 (s, 1H), 2.46 (s, 3H), 2.45 (s, 3H), 1.80 (s, 3H), 1.72 (s, 3H). ESI-MS: m/z 522 (100%) [M+]. Synthesis of L1o. 1-(3,5-Dimethyl-2-thienyl)-2-(4-iodo-3,5-dimethyl-2-thienyl)perfluorocyclopentene (730 mg, 1.40 mmol), tetrakis(triphenylphosphine)palladium(0) (50 mg, 0.07 mmol), and copper(I) iodide (4 mg, 20 μmol) were dissolved in triethylamine (30 mL), followed by the addition of (trimethylsilyl)acetylene (0.6 mL, 4 mmol). The reaction mixture was stirred at 70 °C overnight, which was monitored by thin-layer chromatography (TLC). The mixture was filtered, and the filtrate was then concentrated under reduced pressure. The product was purified by silica gel column chromatography using petroleum ether as the eluent to afford a yellow solid. Yield: 48% (331 mg). 1H NMR (CDCl3, ppm): δ 6.52 (s, 1H), 2.51 (s, 3H), 2.45 (s, 3H), 1.79 (s, 3H), 1.73 (s, 3H), 0.23 (s, 9H). 13C NMR (CDCl3, ppm): δ 148.1, 144.4, 142.1, 141.4, 129.4, 129.2, 123.1, 120.6, 119.7, 99.9 (CC), 98.4 (CC), 15.4 (s, CH3), 15.3 (s, CH3), 14.9 (s, CH3), 14.8 (s, CH3), 0.10 (Si(CH3)3). ESI-MS: m/z 492 (100%) [M+]. IR (KBr): 2149 cm−1 (CC). Synthesis of L2o. A dry THF (40 mL) solution of 3-bromo-2methyl-5-(trimethylsilyl)ethynylthiophene (548 mg, 2 mmol) was cooled to −78 °C. To the solution was slowly added n-butyllithium (1.6 M in hexane, 1.3 mL, 2.1 mmol). Upon stirring at −78 °C for 30 min, (3,5-dimethyl-2-thienyl)perfluorocyclopentene (608 mg, 2 mmol) in dry THF (5 mL) was added to the solution. After further stirring at −78 °C for 2 h, dilute hydrochloric acid was added. The product was extracted with ether, dried with MgSO4, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography using petroleum ether as the eluent to afford the product as a pale-yellow oil. Yield: 49% (468 mg). 1H NMR (400 MHz, CDCl3, ppm): δ 7.21 (s, 1H), 6.52 (s, 1H), 2.44 (s, 3H), 1.90 (s, 3H), 1.68 (s, 3H), 0.25 (s, 9H). 13C NMR (CDCl3, ppm): δ 144.3,

exhibit significant electronic density at the reactive C atoms, only LUMO+1 is phase-matching with bonding character and favorable for photocyclization through formation of the C−C bond. For 2co (Table S5 in the SI), because HOMO−2 → LUMO +1 with 11% contribution represents the only UV absorption (S7 at 335 nm) transition involving LUMO+1, a low conversion yield of 2co → 2cc (18% at the PSS) is reasonable. For 3co (Table S8 in the SI), several UV absorption transitions involving LUMO+1, including HOMO−1 → LUMO+1 (13%), HOMO−8 → LUMO+1 (9%), HOMO−8 → LUMO+1 (35%), and HOMO−6 → LUMO+1 (18%), induce much more conversion of 3co to 3cc (67% at the PSS) than that of 2co to 2cc (18% at the PSS). The 100% conversion of 4co to 4cc or 5co to 5cc is correlated to three or two significant UV absorption transitions involving LUMO+1, including HOMO−8 → LUMO+1 (42%), HOMO−5 → LUMO+1 (27%), and HOMO−6 → LUMO+1 (12%) for 4co (Table S11 in the SI) and HOMO−7 → LUMO+1 (57%) and HOMO−4 → LUMO+1 (15%) for 5co (Table S14 in the SI). Ring-closure absorption for dually ring-closed species 2cc− 4cc is due to electron promotion from HOMO to LUMO, whereas that for 5cc, from HOMO−1 → LUMO and HOMO → LUMO+1. The transition energy of HOMO → LUMO reduces progressively with stepwise photocyclization, following Eoo > Eco > Ecc for complexes 2−4, coinciding with a gradual red shift of ring-closure absorption measured experimentally. For complex 5, the transition energy follows E5oo (3.6 eV) > E5co (2.0 eV) = E5cc (2.0 eV), coinciding with measured ringclosure absorption without a distinct red shift upon stepwise photocyclization.

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CONCLUSIONS L1o−L4o with progressively red-shifted ring-closure absorption were elaborately designed to modulate stepwise photochromism in platinum(II) complexes 1oo−5oo. With the progressive red shift of ring-closure absorption following L1o → L2o → L3o → L4o, stepwise photocyclization is increasingly facilitated in trans-platinum(II) complexes following 1oo → 2oo → 3oo → 4oo. The progressively facile stepwise photocyclization following 2oo → 3oo → 4oo is reasonably interpreted by TDDFT studies, in which singly ring-closed species following 2co → 3co → 4co exhibit gradually enhanced transition character involving LUMO+1, which is the only unoccupied orbital having significant electronic density at the reactive C atoms responsible for further photocyclization at the other ringG

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143.3, 141.3, 132.6, 129.5, 125.1, 121.4, 99.7 (CC), 96.5 (CC), 15.4 (CH3), 15.3 (CH3), 15.2 (CH3), −0.23 (Si(CH3)3). ESI-MS: m/z 479 (100%) [M + 1]+. IR (CH2Cl2): 2149 cm−1 (CC). Synthesis of L3o. This compound was prepared by the same synthetic procedure as that of L2o except for the use of (2,5-dimethyl3-thienyl)perfluorocyclopentene instead of (3,5-dimethyl-2-thienyl)perfluorocyclopentene. Yield: 65% (621 mg). 1H NMR (400 MHz, CDCl3, ppm): δ 7.22 (s, 1H), 6.69 (s, 1H), 2.41 (s, 3H), 1.84 (s, 3H), 1.83 (s, 3H), 0.24 (s, 9H). 13C NMR (CDCl3, ppm): δ 143.2, 139.8, 137.9, 132.3, 125.0, 124.5, 124.3, 121.4, 99.7 (CC), 99.4 (CC), 15.0 (CH3), 14.3 (CH3), 14.2 (CH3), −0.30 (Si(CH3)3). ESI-MS: m/z 479 (100%) [M + 1]+. IR (CH2Cl2): 2149 cm−1 (CC). Synthesis of 1oo. trans-Pt(PEt3)2Cl2 (75 mg, 0.15 mmol) and L1o (157 mg, 0.32 mmol) were dissolved in a degassed CH2Cl2. Tetrabutylammonium fluoride (0.1 mL, 1 M in THF) and CuI (5 mg) were then added to the solution. The reaction mixture was stirred in the dark for 1 day, in which the reaction was monitored by TLC. The product was purified by silica gel column chromatography using dichloromethane−petroleum ether (1:2, v/v) as the eluent. Yield: 65% (124 mg). ESI-MS: m/z 1270 (100%) [M + 1]+. Anal. Calcd for C50H56F12P2PtS4: C, 47.28; H, 4.44. Found: C, 47.52; H, 4.31. 1H NMR (400 MHz, CDCl3, ppm): δ 6.49 (s, 2H), 2.46 (s, 6H), 2.44 (s, 6H), 2.11−2.07 (m, 12H), 1.76 (s, 6H), 1.71 (s, 6H), 1.12−1.08 (m, 18H). 31P NMR (161.97 MHz, CDCl3, ppm): δ 10.98 (JPt−P = 2382 Hz). 13C NMR (CDCl3, ppm): δ 143.8, 142.6, 141.5, 141.2, 129.1, 128.3, 121.1, 118.4, 112.3 (t, JC−P = 14.5 Hz, CC), 102.3 (CC), 16.3 (t, JC−P = 17.5 Hz, CH2P), 15.6 (CH3), 15.3 (CH3), 15.2 (CH3), 15.0 (CH3), 8.2 (CH3CH2P). IR (CH2Cl2): 2096 cm−1 (CC). Synthesis of 2oo. This compound was prepared by the same synthetic procedure as that of 1oo except for the use of L2o (153 mg, 0.32 mmol) in place of L1o. Yield: 70% (130 mg). ESI-MS: m/z 1242 (100%) [M + 1]+. Anal. Calcd for C48H52F12P2PtS4: C, 46.41; H, 4.22. Found: C, 46.75; H, 4.09. 1H NMR (400 MHz, CDCl3, ppm): δ 6.77 (s, 2H), 6.51 (s, 2H), 2.44 (s, 6H), 2.17−2.09 (m, 12H), 1.86 (s, 6H), 1.70 (s, 6H), 1.25−1.17 (m, 18H). 31P NMR (161.97 MHz, CDCl3, ppm): δ 11.21 (JPt−P = 2341 Hz). 13C NMR (CDCl3, ppm): δ 143.8, 141.2, 139.1, 129.3, 127.4, 127.1, 124.6, 120.7, 114.4 (t, JC−P = 14.6 Hz, CC), 100.8 (CC), 16.5 (t, JC−P = 17.6 Hz, CH2−P), 15.4 (CH3), 15.3 (CH3), 14.2 (CH3), 8.3 (CH3CH2P). IR (CH2Cl2): 2092 cm−1 (CC). Synthesis of 3oo. This compound was prepared by the same synthetic procedure as that of 1oo except for the use of L3o (153 mg, 0.32 mmol) in place of L1o. Yield: 64% (120 mg). ESI-MS: m/z (%) 1242 (100) [M + 1]+. Anal. Calcd for C48H52F12P2PtS4: C, 46.41; H, 4.22. Found: C, 46.80; H, 4.15. 1H NMR (400 MHz, CDCl3, ppm): δ 6.78 (s, 2H), 6.69 (s, 2H), 2.41 (s, 6H), 2.16−2.10 (m, 12H), 1.85 (s, 6H), 1.79 (s, 6H), 1.24−1.16 (m, 18H). 31P NMR (161.97 MHz, CDCl3, ppm): δ 11.22 (JPt−P = 2338 Hz). 13C NMR (CDCl3, ppm): 139.7, 138.9, 137.5, 127.3, 126.8, 124.7, 124.6, 120.7, 114.4 (t, JC, P = 13.9 Hz, CC), 100.8 (CC), 16.5 (t, J(C, P) = 17.6 Hz, CH2P), 15.1 (CH3), 14.3 (CH3), 14.2 (CH3), 8.3 (CH3CH2P). IR (CH2Cl2): 2092 cm−1 (CC). Synthesis of 4oo. This compound was prepared by the same synthetic procedure as that of 1oo except the use of L4o (167 mg, 0.32 mmol) in place of L1o. The product was purified by column chromatography using dichloromethane−petroleum ether (2:1, v/v) as the eluent. Yield: 75% (153 mg). ESI-MS: m/z 1368 (100%) [M + 1]+. Anal. Calcd for C56H54F12N2P2PtS4: C, 49.16; H, 3.98; N, 2.05. Found: C, 49.06; H, 4.03; N, 1.98. 1H NMR (400 MHz, CDCl3, ppm): δ 8.54 (d, J = 4.82 Hz, 2H), 7.69 (td, J1 = 7.92 Hz, J2 = 1.72 Hz, 2H), 7.60 (d, J = 8 Hz, 2H), 7.51 (s, 2H), 7.18−7.14 (m, 2H), 6.78 (s, 2H), 2.14−2.06 (m, 12H), 1.99 (s, 6H), 1.87 (s, 6H), 1.23−1.14 (m, 18H). 31P NMR (161.97 MHz, CDCl3, ppm): δ 11.27 (JPt−P = 2337 Hz). 13C NMR (CDCl3, ppm): δ 151.7, 149.5, 144.2, 142.5, 139.0, 136.7, 127.6, 126.7, 126.0, 124.2, 124.0, 122.2, 118.5, 114.8 (t, JC−P = 14.6 Hz, CC), 100.6 (CC), 16.5 (t, JC−P = 17.7 Hz, CH2P), 14.8 (CH3), 14.4 (CH3), 8.3 (CH3CH2P). IR (CH2Cl2): 2092 cm−1 (C C). Synthesis of 5oo. This compound was prepared by the same synthetic procedure as that of 1oo except for the use of cis-

Pt(PEt3)2Cl2 (75 mg, 0.15 mmol) and L4o (167 mg, 0.32 mmol) in place of trans-Pt(PEt3)2Cl2 and L1o. Yield: 60% (123 mg). ESI-MS: m/z 1368 (60%) [M + 1]+, 1391 (100%) [M + Na]+. Anal. Calcd for C56H54F12N2P2PtS4: C, 49.16; H, 3.98; N, 2.05. Found: C, 49.38; H, 3.91; N, 2.01. 1H NMR (400 MHz, CDCl3, ppm): δ 8.53 (d, J = 4.68 Hz, 2H), 7.68 (td, J1 = 7.72 Hz, J2 = 1.72 Hz, 2H), 7.60 (d, J = 8 Hz, 2H), 7.49 (s, 2H), 7.16−7.12 (m, 2H), 6.98 (s, 2H), 2.04−1.98 (m, 12H), 1.97 (s, 6H), 1.85 (s, 6H), 1.18−1.10 (m, 18H). 31P NMR (CDCl3, ppm): δ 4.78 (JPt−P = 1549 Hz). 13C NMR (CDCl3, ppm): δ 151.7, 149.5, 144.4, 142.5, 139.5, 136.7, 128.3, 127.2, 126.0, 124.1, 124.0, 122.1, 118.5, 116.0, 98.1, 53.4, 17.2 (t, JC−P = 19.0 Hz, CH2P), 14.8 (CH3), 14.4 (CH3), 8.4 (CH3CH2P). IR (CH2Cl2): 2107 cm−1 (CC). Physical Measurements. 1H, 13C, and 31P NMR spectra were performed on a Bruker Avance III (400 MHz) spectrometer with SiMe4 as the internal reference and H3PO4 as the external reference. UV−vis absorption spectra were measured on a Perkin-Elmer Lambda 25 UV−vis spectrophotometer. IR spectra were recorded on a Magna 750 FT-IR spectrophotometer. Elemental analyses (C, H, and N) were carried out on a Perkin-Elmer model 240 C elemental analyzer. Electrospray ionization mass spectrometry (ESI-MS) was recorded on a Finnigan DECAX-30000 LCQ mass spectrometer. UV light was produced using a ZF5 UV lamp (365 nm), and visible-light irradiation was carried out using a LZG220 V 1 kW tungsten lamp with cutoff filters. The quantum yields were determined by comparing the reaction yields of the diarylethenes relative to 1,2-bis(2-methyl-5phenyl-3-thienyl)perfluorocyclopentene.53 Cyclic voltammetry and differential pulse voltammetry were measured using a model 263A potentiostat/galvanostat in dichloromethane solutions containing 0.1 M (Bu4N)(PF6) as the supporting electrolyte. Platinum and glassy graphite were used as the counter and working electrodes, respectively, and the potential was measured against the Ag/AgCl reference electrode. Theoretical Methodology. All calculations of complexes 1oo/ co−5oo/co and 2cc−5cc were implemented in the Gaussian03 program package. 54 The DFT55a with the gradient-corrected correlation functional PBE1PBE55b was first used to optimize the geometrical structures in the ground states. In the optimization processes, the convergent values of maximum force, root-mean-square (rms) force, maximum displacement, and rms displacement were set by default. Then, 100 singlet excited states of these investigated complexes in a dichloromethane solution were calculated by the TDDFT56 method based on the optimized geometrical structures. The solvent effects were taken into account by the polarizable continuum model method.57 The self-consistent-field convergence criteria of the rms density matrix and maximum density matrix were set at 10−8 and 10−6 au, respectively, in all of these electronic structure calculations. The iterations of excited states continued until the changes on the energies of states were no more than 10−7 au between iterations, and then convergences were reached in all of the excited states. In these calculations, the Lanl2dz effective core potential was used to describe the inner electrons of Pt, S, and P atoms, while its associated double-ζ basis set of Hay and Wadt was employed for the remaining outer electrons.58 Other nonmetal atoms of F, N, C, and H were described by the all-electron basis set of 6-31G(p,d).59 To precisely describe the molecular properties, one additional f-type polarization function was employed for the Pt atom (αf = 0.18).60 Visualization of the optimized structures and frontier molecular orbitals was performed by GaussView. The Ros and Schuit method (C-squared population analysis method)61 was supported to analyze the partition orbital composition using the Multiwf n2.4 program.62

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ASSOCIATED CONTENT

* Supporting Information S

Tables and figures giving additional spectroscopic and computational data. This material is available free of charge via the Internet at http://pubs.acs.org. H

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the NSFC (Grants 20931006, U0934003, 91122006, and 21303204) the 973 project (2014CB845603) from MSTC and the NSF of Fujian Province (Grant 2011J01065).

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